Topoisomerase II is an essential enzyme that is required for proper chromosome structure and segregation, and plays important roles in DNA replication and recombination. The enzyme relaxes DNA and removes knots and tangles from the genome by passing an intact double helix through a transient double-stranded break that it generates in a separate DNA segment. Proliferating eukaryotic cells cannot survive without topoisomerase II. However, since the enzyme must create double-stranded breaks in the genetic material in order to carry out its catalytic functions, it also poses an intrinsic threat to genomic integrity every time it acts. Beyond its critical cellular functions, topoisomerase II is the primary target for some of the most important drugs used for the treatment of human cancers. These agents elicit their cytotoxic effects by a mechanism that is markedly different than that of other drugs. Rather than inhibiting the catalytic activity of the enzyme, anticancer drugs dramatically increase levels of covalent topoisomerase II-cleaved DNA complexes that are normal, but fleeting, intermediates in the catalytic cycle of the enzyme. When the resulting enzyme-associated DNA breaks accumulate, they generate mutations, chromosomal translocations, and ultimately trigger cell death pathways. Anticancer drugs targeted to the type II enzyme are referred to as topoisomerase H poisons because they convert this enzyme to a potent physiological toxin that generates DNA damage in treated cells. Although topoisomerase II is one of the most important targets for cancer chemotherapy, evidence suggests that it also has the potential to trigger the disease. Indeed, secondary leukemias are observed in some patients treated with topoisomerase II-targeted drugs and specific infant leukemias are associated with maternal exposure (during pregnancy) to natural and environmental topoisomerase II poisons. Most of these patients carry translocations involving an 8.3 kb breakpoint cluster in the MLL oncogene at chromosome band 11q23. Despite the central importance of topoisomerase II to cancer, interactions between the enzyme, DNA, and anticancer drugs (or other poisons) have not been well characterized. Thus, the ultimate goals of this proposal are to further delineate the mechanism by which topoisomerase II carries out its essential catalytic reactions, the mechanism by which drugs and other topoisomerase II-active chemicals alter the catalytic function of the enzyme, and the cellular responses to enzyme-generated DNA breaks. Research models for this study will be human topoisomerase II alpha and beta, and yeast (Saccharomyces cerevisiae) and Chlorella virus topoisomerase II.